As will be appreciated herein below, except as otherwise indicated, aluminium alloy designations and temper designations refer to the Aluminum Association designations in Aluminum Standards and Data and the Registration Records, as published by the Aluminum Association in 2014 and well known to the persons skilled in the art.
The industrial scale automotive sheet production of the heat-treatable AlMgSi-alloy series, also known as 6000-series aluminium alloys, typical examples include AA6005, AA6014, AA6016 and AA6022, comprises several discrete steps, namely semi-continuous direct chill (DC)-casting or electromagnetic casting (EMC-casting) of a rolling slab or ingot, although on a less preferred basis also continuous casting like belt or roll casting can be applied, preheating of the slab at about 500° C. to 580° C. for several hours for homogenization of the microstructure, hot rolling into hot rolled strip at a gauge of about 3 to 12 mm, the hot rolled strip is typically hot coiled and cooled down to ambient temperature, the hot rolled strip is cold rolled to final gauge in several passes, optionally an intermediate anneal is applied prior to the cold rolling or during the cold rolling process, and at final gauge the strip is annealed to adjust the required material properties. The annealing can be done either in a continuous annealing furnace or in a batch type furnace.
Unfortunately, sheet products produced in this way from semi-continuous cast ingots often suffer from a phenomenon known as roping, ridging or “paint brush” line formation (the term “roping” is used henceforth), i.e. the formation of narrow bands having a different crystallographic structure than the remaining metal resulting from the metal rolling operation and generally aligned in the direction of rolling. During subsequent transverse straining of the sheet products as they are being formed into automotive parts, these bands manifest themselves as visible surface undulations, which detract from the final surface finish of the automotive product.
Roping has been encountered by many others in this art, and it has been found that roping may be inhibited by modifying the sheet production method so that recrystallisation occurs at an intermediate stage of processing. The inhibition of roping is addressed, for example, in U.S. Pat. No. 5,480,498 (Reynolds), U.S. Pat. No. 4,897,124 (Sky Aluminum), and also in U.S. Pat. No. 6,120,623 (Alcan). In these patents, roping is controlled by introducing a batch annealing step (e.g. heating at a temperature within the range of 316° C. to 538° C.) at an intermediate stage of the sheet product formation, e.g. after hot rolling but before cold rolling, or after an early stage of cold rolling.
An economical attractive method of producing 6000-series aluminium sheet material is by means of continuous annealing at final gauge. At the end of a continuous annealing furnace, the strip material is rapidly cooled or quenched to ambient temperature, for example by means of forced air cooling or spray cooling systems. By this solution anneal the main alloying elements Mg and Si are dissolved, leading to a good formability, control of the yield strength and bake hardening behaviour, and brings the sheet material to a T4 temper.
In order to produce AlMgSi-alloy sheet material on an industrial scale in an economical attractive manner, it is required that sufficiently high line speeds can be maintained while the sheet material is moving through the continuous annealing furnace. However, too high line speeds may impact on the soaking time of the strip material at the required annealing temperature and thereby affecting, amongst others, the mechanical properties of the aluminium strip.
There is consequently a need for an improved process of producing aluminium automotive alloy sheet products that exhibit little or no roping while maintaining desirable T4 characteristics.